The TonB system in Escherichia coli and other Gram-negative bacteria is a hot target for designer antibiotics: it’s important for iron acquisition and virulence and it transports nutrients across the outer membrane. But a study released by mBio this week reveals that what we thought we knew about the structure of TonB is wrong. Postle et al. took a closer look at the crystal structures of the dimers that make up TonB and by making amino acid substitutions at supposedly critical sites, determined that the structure in crystal form can’t be the same as the structure in vivo. The authors speculate that the crystal structures get it wrong because they were obtained in the absence of the TonB transmembrane domain, cytoplasmic membrane proteins ExbB and ExbD, and/or the proton motive force.